The present invention relates to methods for measuring proportions of suicides, methods for measuring annealing temperatures, methods for fabricating semiconductor devices and x-ray photo receivers.
With recent downsizing of semiconductor devices, patterns of LSIs (Large Scale Integration devices) have become finer and finer. Techniques for operating such Large Scale Integration devices with fine patterns at high speed have also been needed. A delay time in an Large Integration devices is mainly determined by the product of a resistance and a capacitance. That is, the smaller the resistance or the capacitance is, the faster the Large Scale Integration device operates. Accordingly, techniques for reducing the resistance of gate electrodes and reducing the resistance of source and drain regions are necessary for increasing the speed of Large Scale Integration devices.
To reduce the resistance, suicides (compounds of silicon with metal, e.g., tungsten silicide) having lower resistances than conventional polysilicon electrodes are used for gate electrodes and source and drain regions. In addition, silicides using titanium, cobalt and nickel as their metal components and utilizing self-alignment, called a SALICIDE process (self-align silicide), have been intensively studied. In particular, the SALICIDE process using titanium or cobalt has been already used in mass production to reduce the resistances of gate electrodes and source and drain regions.
The silicides are formed through reaction between silicon and metal. However, even one metal element can be used to produce a plurality of types of silicides having different combining ratios of silicon and metal. Silicide reactions using cobalt, platinum, titanium and nickel are expressed as follows:2X+Si→X2SiX+Si or X2Si+Si→XSiX+2Si, X2Si+3Si or XSi+Si→XSi2where X denotes the metals. These reactions mainly produce three types of silicides. The first two reactions occur in a region at a relatively low temperature. With respect to silicides using cobalt and titanium, the specific resistances of two silicides produced by these reactions are larger than the specific resistance of the third silicide produced by the last reaction. Accordingly, the third reaction is finally caused to form a low-resistance silicide. On the other hand, with respect to silicides of nickel, silicides produced by the first two reactions have substantially the same specific resistance, which is smaller than the specific resistance of the third silicide produced by the last reaction. Accordingly, reaction is terminated after the first two reactions. With respect to platinum, reaction terminates after the first two reactions.
A process of forming a cobalt silicide in the surface of gate, source and drain in a fabrication process of a MOS transistor will be described as an example of formation of such silicides.
First, as shown in FIG. 10A, a gate oxide film (not shown) with a thickness of about 3 nm is formed by thermal oxidation in the surface of part of a silicon substrate 401 separated by a shallow trench isolation 402. Subsequently, a polysilicon layer 403 is deposited by a CVD process to a thickness of about 150 nm thereon.
Next, as shown in FIG. 10B, ions of at least one of boron, indium, phosphorus, arsenic and antimony are implanted into the polysilicon layer 403 as an impurity, thereby defining a doped layer. Then, the doped layer is patterned to form a gate electrode 405. Thereafter, a region between the gate electrode 405 and the shallow trench isolation 402 is doped with ions as an impurity, thereby forming an LDD layer 406 as a shallow doped layer.
Then, a silicon oxide film is deposited by a CVD process to a thickness of about 100 nm, and anisotropic etching is performed on the silicon oxide film to expose the gate electrode 405 so that part of the silicon oxide film remains as a sidewall 407 as shown in FIG. 10C. Then, a region between the sidewall 407 and the shallow trench isolation 402 is doped with ions as an impurity, thereby forming a deeply-doped layer. Then, the deeply-doped layer is subjected to heat treatment to be activated. In this manner, source and drain layers 409 and 410 with an LDD structure are formed in an upper part of the silicon substrate 401 adjacent to the gate electrode 405.
Thereafter, a silicon oxide film (natural oxide film, not shown) on the surfaces of the gate electrode 405, the source layer 409 and the drain layer 410 is removed with buffered hydrofluoric acid, and then a cobalt film with a thickness of about 10 nm is formed thereon. Then, a titanium nitride film 412 with a thickness of about 20 nm for preventing oxidation of cobalt is formed thereon as shown in FIG. 10D. Thereafter, a first rapid thermal annealing (RTA) is performed at 550° C. for 30 seconds to cause a reaction between cobalt and silicon, thereby forming a cobalt silicide layer 413. The cobalt silicide formed at this time mainly includes Co2Si and CoSi.
Subsequently, as shown in FIG. 10E, unreacted cobalt and the titanium nitride film 412 are removed. Then, a second RTA is performed at 850° C. for 30 seconds, thereby further reducing the resistance of the cobalt silicide layer 413 formed on the surfaces of the gate electrode 405, the source layer 409 and the drain layer 410 to obtain a silicide having a desired composition as a final product. That is, the reaction proceeds to make most of the cobalt silicide into CoSi2.
In the foregoing example, the formation of the cobalt silicide layer 413 has been described. However, if titanium is used, suicides are formed with substantially the same procedure though annealing temperature regions for the RTAs are different.
In the case of formation of nickel suicides, the specific resistances of Ni2Si and NiSi do not differ extremely and NiSi2 formed by annealing at 600° C. or more has a large specific resistance. Therefore, RTA is normally performed at a temperature in the range from about 400° C. to about 500° C. in the formation of nickel suicides.
Examples of measurement techniques regarding silicide include a technique described in Journal of Vacuum Society, Technology B, vol. 17, p2284 (1999), which will be detailed later.
With the foregoing processes, silicides using various types of metals are formed. With the reduction of pattern width of LSIs, tungsten as a metal element for silicides has been conventionally replaced with titanium and cobalt in order to further reduce the resistance. However, when the pattern width is reduced to 0.1 μm or less, there arise a problem of a break due to migration or the like in silicides and a problem of failure in obtaining a desired low resistance. To cope with these problems, using various metal elements and adding another material have been studied, but have not been put into actual use yet.
As another solution of the problems, a method of adjusting conditions for the first and second RTAs to make a silicide as a final product in an optimum state with low resistance and high migration resistance can be used. To obtain the silicide in this optimum state, it is necessary to know the proportions of a plurality of types of silicides having different compositions after the first RTA and it is also necessary to set conditions for the second RTA in accordance with the proportions of the silicides. In addition, it is extremely difficult to set the conditions for the second RTA if the proportions of the suicides having different compositions after the first RTA are not within an appropriated range. Accordingly, it is also necessary to control conditions for the first RTA such that these proportions are within the appropriate range. Among the conditions for the first RTA, it is important to strictly control the annealing temperature, i.e., the temperature which a silicon substrate reaches in the first RTA. This is because no silicide with a desired resistance is obtained finally unless this temperature is controlled within the variation of ±5° C.
In other words, the measurement of the three proportions of X2Si, XSi and XSi2, respectively, after the first RTA, and the control of the temperature in the first RTA, are important. With respect to the former, the proportions have been estimated using sheet resistance measurement to date. As a new method, measurement using a spectroscopic ellipsometry which is an optical technique has been studied (e.g., in Journal of Vacuum Society, Technology B, vol. 17, p2284(1999)). On the other hand, with respect to the latter, temperature measurements by measuring a change in thickness of a silicon oxide film or a change in the sheet resistance after ion implantation, direct temperature measurement with a thermocouple, and other methods have been studied.
With the sheet resistance measurement as the former measurement technique, the proportions of the respective silicides are estimated using the fact that X2Si, XSi and XSi2 have respective specific resistances. The total amount of metal can also be measured by measuring a peak intensity of fluorescence X-rays from metal such as cobalt with an x-ray fluorescence measurement.
With the measurement of a change in silicon oxide film thickness as the latter measurement technique, the annealing temperature is measured, using the fact that the silicon oxide film has its thickness increased when annealed in an oxidation ambient. With the measurement of a change in the sheet resistance after ion implantation, annealing on the silicon substrate doped with ions activates the ions so that the sheet resistance changes. It is possible to know the annealing temperature utilizing this phenomenon.
However, in the case of measuring the sheet resistance, the proportions cannot be measured directly and the specific resistance also changes depending on the crystallinity of suicides and the like, so that it is extremely difficult to calculate the proportions of the silicides with high accuracy. In addition, since the sheet resistance is measured by a 4-point probe method, the measurement is easily performed on a silicon substrate or the like, but is not easily performed on a device with a complicated pattern in performing in-line measurement in fabrication process steps. The spectroscopic ellipsometry allows measurement of the proportions of the silicides theoretically, but has not been put into practical use yet.
Since the first RTA is performed at a relatively low temperature of about 500° C., the thickness of the silicon oxide film and the sheet resistance after ion implantation vary slightly even after the first RTA. Accordingly, it is very difficult to accurately measure the temperature in the first RTA using the change in thickness of the silicon oxide film or the change in sheet resistance. In addition, since the thermocouple measures the temperature of a specific part of annealing apparatus, the temperature of the silicon substrate cannot be measured accurately. The annealing temperature may be measured with a thermocouple attached to the silicon substrate at the first use of new annealing apparatus. However, this technique is not suitable for mass production. Therefore, no techniques have been found to date in accurately measuring the annealing temperature in the first RTA in fabrication.